1. Field of the Invention
This invention relates to a process of forming a refractory mass on a surface by spraying from an outlet of a lance and against that surface a mixture of oxidisable particles and refractory particles in a comburent carrier gas so that on combustion of said oxidisable particles, sufficient heat is generated to soften or melt at least the surfaces of the refractory particles to bring about the formation of the refractory mass. The invention also relates to apparatus for forming a refractory mass on a surface by spraying against that surface a mixture of oxidisable particles and refractory particles in a comburent carrier gas so that on combustion of said oxidisable particles, sufficient heat is generated to soften or melt at least the surfaces of the refractory particles to bring about the formation of the refractory mass, which apparatus comprises means for mixing said particles with a carrier gas stream, a lance with an outlet from which they are to be sprayed, and a feed line for conveying the carrier gas and entrained particles to the lance outlet.
Such processes are useful for forming refractory coatings on refractory blocks and other surfaces, are especially suitable for repairing or strengthening furnace linings in situ, and can in some cases be used while the furnace is still operating. The processes are particularly apt for use in the repair of erosion caused by contact between refractories and molten metal, such as in furnaces, ladles and convertors used in the iron and steel industries.
2. Description of the related Art
Among previous proposals in this field are those set forth in Patent Specification Nos GB 1 330 894 (Glaverbel) and GB 2 035 524 A (Coal Industry [Patents] Limited).
As is well known, the refractory particles are chosen to confer the desired refractory properties on the mass to be formed, for example to match the chemical composition of a refractory substrate against which they are to be sprayed, or to form a higher quality refractory surface on that substrate. As oxidisable material, it is most usual to use silicon and/or aluminium particles, though particles of other materials such as magnesium and zirconium may be used where it is desired to impart special properties to the refractory mass to be formed. Of course there are other materials which could be used, but these are in general less preferred. It has been recommended to use oxidisable particles having a mean grain size below 50 .mu.m or even below 10 .mu.m (GB 1 330 894 A).
It is of course clearly desirable to ensure that sufficient oxygen is available for the desired extent of combustion, and the supply of a substantial excess of oxygen has been recommended. For example, GB 1 330 894 A recommends using oxygen as carrier gas, and in its Examples, specifies hourly feed rates of 60 kg mixed particles in 1200L oxygen and 30 kg mixed particles in 480L oxygen.
It is generally desirable that the refractory mass formed should contain substantially no still-oxidisable material, since the presence of such material usually detracts from the quality of that refractory mass, and entails that the unburnt material will not have been able to yield heat during spraying so that it is to that extent wasted. This would add unnecessarily to the cost of the process. Since stilloxidisable material can hardly burn when it is buried in the refractory mass being formed, it must burn during its trajectory, or while it is exposed on the surface being sprayed. In use, the outlet at the tip of the lance from which the material is sprayed is often held at a distance of some 10 to 30 cm from the surface on which the refractory mass is being formed, and it is accordingly desirable that the oxidisable material should burn rather rapidly. Such rapid burning is promoted by the use of very small oxidisable particles which are well mixed in an oxygen rich gas stream.
It is also desirable, to promote durability of the refractory mass formed, that the refractory mass should be free from porosity, especially if the refractory will be in contact with molten metal during its working life. The risk of forming a porous refractory mass is increased when large quantities of carrier gas are used.
Feeding very small oxidisable particles well mixed in an oxygen rich gas stream is most beneficial for rapid and efficient combustion on discharge from the lance: however this can also give rise to conditions under which combustion can be supported within the feed line leading to the lance outlet. This would clearly halt the process, and could lead to damage to the apparatus used. Such combustion may in some circumstances be initiated by flashback from the lance outlet if the speed of flame propagation is greater than the speed at which the material is ejected from the lance. The risk of combustion within the feed line is increased by the use of very small oxidisable particles, by increasing the weight proportion of oxidisable particles in relation to the proportion of refractory particles, by increasing the proportion of oxygen in the carrier gas stream and by increasing the diameter of the feed line. Flashback may take a relatively mild form, leading merely to blockage of the lance outlet, or it may be more serious, going right back to the point where the particles are mixed with the oxygen carrier stream. For that reason. GB 1 330 894 A recommends the use of an apparatus incorporating various safety features as set forth in GB 1 330 895 A, also in the name of Glaverbel.
GB 2 035 524 A proposes to overcome the problem of flashback by feeding the mixture of particles in a carrier gas which will not support oxidation of the oxidisable particles (air is recommended), and supplying oxygen to the lance adjacent its outlet. An hourly feed rate of 30 kg mixed particles in 3000 to 6000L air with the supply of oxygen at a volume rate of 2 to 4 times that of the air is recommended and exemplified. Clearly, no flash will be able to propagate back in a carrier gas which will not support oxidation. Further, by the choice of somewhat larger oxidisable particles, up to 152 .mu.m, that specification suggests that the problem of lance tip blockage can be reduced. Indeed, it is stated that combustion of the mixture does not start for some distance from the lance, where sufficient mixing of the oxygen with the mixed particles is attained. Accordingly, there is a risk that unburnt oxidisable material will be incorporated in the refractory mass formed. Also the use of such large quantities of gas in relation to the quantity of particles used tends to promote the formation of a porous refractory mass.
Material feed rates as specified in those prior specifications entail rather low rates of build up of the refractory mass to be formed. In order to achieve a substantial increase in the build-up rate of the refractory material it is necessary either to use more than one feed line for the lance, which is inconvenient, or to increase the feed line diameter, so that it can accommodate a greater flow of the particle mixture. The use of a larger diameter feed line also tends to increase the risk of combustion within the feed line, since it is easier for a flame to propagate in a larger diameter pipe.
Apart from flashback from the lance outlet, there is another important potential cause of combustion within a feed line. It will be appreciated that as the particles are carried along they will collide with each other and with the walls of the feed line This will generate heat and at high carrier gas and particle velocities, which are desirable to enable rapid build up of the refractory mass being formed, this heat can be sufficient to induce spontaneous combustion of the oxidisable particles, especially when they are carried in a stream which is very rich in oxygen.